Following the swift development of wireless communication technology and the increasing demand for communications, the fifth generation of communication technology (5G) is fast approaching. An important application of 5G is Machine-Type Communications (MTC), which, with their large-scale potential for application, have garnered attention from a large number of mobile network operators, equipment manufacturers, and research institutes. MTC applications such as Smart Grid, Intelligent Transportation, Smart Home (Home Automation), remote monitoring, and wireless censoring networks make up an important part of the burgeoning Internet of Things (IoT).
Services that support MTC devices are varied. Requirements for most MTC equipment usually include low cost and low power usage, such as the equipment used for environmental monitoring or the large-scale sensors used in agriculture. Operations that support MTC devices typically do not have strict requirements with regard to time delays, but could have sensitive materials that require high speed transmission.
Fire alarm equipment, for example, includes the following services: Periodic Life-Saving Report Information does not have high time-delay requirements, and its data is as little as tens of BYTES; Fire Alarm Report Information has relatively higher time-delay requirements; its data quantities can be as small as tens of bytes or as large as a Megabyte (M) because video information might need to be reported; Server Commands have relatively higher time-delay requirements and data quantities as low as tens of bytes or even lower; Software version updates have relatively low time-delay requirements and relatively large data quantities, as much as a few Megabytes.
From this we see that the services supporting MTC equipment are varied and comprise, for example, periodic service reporting, anomaly reporting, network command issuance, and software version updates. The Quality of Service that different operations demand of the network are also different.
At the same time, some MTC equipment can transmit high-speed data. In portable applications, for example, Smart Glasses could require the transmission of video in real time, and Smart Watches could require the ability to make phone calls. These services would require greater QoS guarantees.
The quantity of MTC equipment could be enormous, and large-scale equipment access could bring large-scale signal expenditure to the network. In order to effectively lower the cost of MTC equipment and system expenditure of the network-side, the related technologies of the Third Generation Partnership Projects (3GPP) have led to the following two methods of small data transmission for optimization: the control plane optimization mode: the data packet is packaged into a non-Access Stratum (NAS) Protocol Data Unit (PDU) attached to the control plane signal for transmission; the user plane optimization mode: the terminal and the network-side use Data Radio Bearer (DRB) for data transmission; after data transmission is complete, both parties store the bearer information context and the Access Stratum (AS) security context, etc. When data is transmitted once more, both parties resume the use of DRB through the suspend-activate flow.
Each of these small data transmission methods has its pros and cons. The benefits of the control plane optimization mode include: the conservation of signaling, system expenditure, and terminal power usage. However, within the control plane optimization mode exist the following deficits: (1) Safety performance can be somewhat poor. Because no DRB response is established in the control plane optimization program, neither the network-side nor the terminal have the AS security context, and there is no way to implement AS layer encryption on the data. (2) It is not very suitable for the transmission of relatively large quantities of data, considering that the NAS PDU attached to the signal is very limited. As a result, the length of the transmitted data packet is limited. Relatively larger data packets must be divided into relatively more smaller packets. Since MTC equipment typically has poor coverage, packets are easily lost. As a result, it is difficult for the receiver to receive a complete packet. Furthermore, control plane optimization utilizes signaling for data transmission; too many small packets will bring too much signal transmission, which will occupy too many resources. (3) Low-priority data services occupy too many scheduling resources. Typically, the priority of periodic report data transmitted by MTC equipment is relatively low, but because this data is attached to the layer 3 air interface signal transmission, it receives the same scheduling priority of the typical Radio Resource Control (RRC) signal and thus encroaches on the scheduling priority of actual RRC access signals (RRC signaling transmitted without MTC data). This could result in more Air Interface failures. Besides this, when the network simultaneously supports data utilizing the control plane optimization mode for transmission and the DRB for transmission, because the priority level of the Signaling Radio Bearer (SRB) is always greater than that of the DRB, the low-priority levels can always gain higher scheduling priority levels than that of DRB so that they are processed first. This could result in being unable to guarantee the high-priority service scheduling of DRB transmission.
The user plane optimization mode has the following advantages: encryption is relatively good. Since the AS security context is retained, the system can implement AS layer encryption on the data. Encryption results have better guarantees than NAS layer encryption; support for the transmission of relatively larger rates or large packets of data is relatively good. However, the user plane optimization mode has the following deficits: (1) The result of signaling optimization is not as good as that of the control plane optimization mode. The control plane optimization mode can conserve more signaling, whereas the user plane optimization mode must first recover the bearer context information before transmitting data. Relatively speaking, the result of signal optimization does not compare to that of the control plane optimization mode. (2) Bearer context maintenance overhead is relatively large, and there is the risk of skewing.
Because the terminal and the network-side always maintain the terminal MTC service bearer context, there exists a definite system expenditure comprising internal expenditure and Central Processing Unit (CPU) expenditure. Beyond this, contextual skewing may occur with long-term use. Since MTC equipment is normally used for a long period of time, with life spans ranging from a few years to ten or more years, and since the network-side exists in a constant state of flux, there is the risk of terminal contextual differences. In addition, changes to the equipment access cells lead to additional context transfer overhead. Due to the relation of the allocations and sites of the service bearer context, when a site or cell accessed by the equipment changes, the network-side must either deliver or modify the context between the sites.
Additionally, when the NarrowBand-Internet of Things (NB-IoT) data transfer method (i.e., the control plane optimization mode or the user plane optimization mode) transmits big data, there exists the problem of time-delays being too big. Because NB-IoT cells use 200 k narrowband technology, their transmission rate is extremely low; according to the data of TR45.820, its speed is about 160 bit/s; for a 1600 byte software version update, 80 seconds are required before complete transmission is possible. Such a long transmission period will result in the blocking of terminal signal data, and risks process failure. At the same time, if the terminal needs to report video data, there is no way to fulfill video QoS with this long of a time-delay.
The 3GPP network provides NB-IoT data transmission methods (i.e., control plane optimization mode or user plane optimization mode) for MTC equipment, but merely providing a single data transmission method for MTC equipment does not fulfill the different QoS requirements of MTC service transmission scenarios when the device is met with a rush of big data. A single mode could also negatively affect the network by, for example, causing too much expenditure on the network-side.